Pinhole camera question

Discussion in 'Pinhole Photography' started by JHannon, Jul 6, 2003.

  1. JHannon

    JHannon Member

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    Hi all, I built a simple pinhole camera with my son made out of cardboard that uses 4X5 sheet film. The distance from the pinhole to the film is about 6 inches. I have been having trouble getting good images with my hand punched pinholes (sewing needle) exposure is OK but the focus seems poor. I am pretty careful to keep it steady.

    Is there are way to get more precise pinholes other than using a sewing needle? I may be mis-calculating the pinhole diameter needed for the focal distance.

    Thanks in advance for your advice.

    John
     
  2. glbeas

    glbeas Member

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    Are you just pushing the pin through or using a piece of wood to back up the metal as you punch it? You should also deburr the hole very carefully with some very fine grit sandpaper.
     
  3. Nige

    Nige Subscriber

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    also, what material are you using for the pinhole? if you'r eusing the cardboard the box is made from you're going to have a hard time getting a 'sharp' image. I make mine out of aluminium 'coke' cans. Cut a can up with some scissors and make the hole by swiveling the needle (drilling) rather than punching it through, and like Gary says, sand each side with the finest sandpaper your can lay your hands on.
     
  4. JHannon

    JHannon Member

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    Nige & glbeas, thanks for your response. I do use a cut coke can and sand down the burrs on both sides. I think the pinhole diameter is wrong. I think it should be .298mm for the 152mm focal length. Any idea what size needle to use?

    Thanks,
    John
     
  5. glbeas

    glbeas Member

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    As a needle has a good bit of taper to it you should be able to use a hard piece of wood behind it to restrict the penetration and make a smaller hole as needed. Simple get a larger needle for bigger holes. If you have or know someone with a set of calipers you might could measure the needle diameter to get an idea how big the hole is going to be. If doing a smaller hole than the shaft of the needle the measure how far up the taper of the point the right size is and try to aim for that depth when "drilling" the hole.
     
  6. Nige

    Nige Subscriber

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    have you measured your pinhole to confirm the 0.298mm?
     
  7. Aggie

    Aggie Member

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    ..
     
  8. Lemastre

    Lemastre Member

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    Project the pinhole as large as you can get it with your slide projector or enlarger and ratio the diameter of the projected image with something of known length projected to the same enlargement. This gives the actual pinhole diameter. The diameter of the pinhole is not all that critical as long as it's very small and reasonably clean-edged.
     
  9. Ed Sukach

    Ed Sukach Member

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    I don't think this would work - the theory behind the "pinhole" is to diffract the light passing through to the extent that it would give a "lens- like" effect... bending the rays to a "pseudo" focus plane. I've tried measuring small spaces on a Optical Comparator, and the edges become *extremely* fuzzy and indistinct. If they did not, it is likely that the "pinhole" is too large for use.
     
  10. Nige

    Nige Subscriber

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    I've used my enlarger and it worked really well.
     
  11. Ole

    Ole Moderator Staff Member Moderator

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    Not quite, Ed...

    The theory is that light moves only in straight lines through air - excepting diffraction. so a light ray from upper left would hit the lower right corner of the film, and nothing else. Moving the pinhole in and out affects only the effective focal length (and thus aperture), not focus.

    Diffraction is what kills sharpness in pinhole photography, otherwise a smaller hole would always give sharper pictures.
     
  12. Ed Sukach

    Ed Sukach Member

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    Oh, I agree with the "not quite". I will point out that the study of diffraction and the way the wave propagation affects the light ray paths is complex and would require a "course of study" FAR beyond the scope of concise messages here. That is why I tried to cover my gluteous maximus by using the "pseudo" before the "focus". The "pseudo" does not appear to have been very successful.

    BTW .. "diffraction kills sharpness" - is true - but without it, "pinhole" would not work at all.
     
  13. Ole

    Ole Moderator Staff Member Moderator

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    To the contrary, Ed: Without diffraction, pinholes would give perfect images!
     
  14. Ed Sukach

    Ed Sukach Member

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    *MOST* interesting. If not by diffraction, how does a "pinhole" cause an image to be formed on a plane?

    If we increase the size of the pinhole, we will decrease diffraction. There must be some limit ot the increase. Where?
     
  15. Ole

    Ole Moderator Staff Member Moderator

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    The "image" is formed by the light moving in straight lines. So the resolution is inversely proportional to the size of the hole. Same as with lenses; just about anything gives a decent enough image at f:90 while it could be wholly unacceptable at full opening. Now imagine a really, really dreadful lens. Stop down to f:360 or so, and the image will be reasonably sharp and DOF really great. Then discard those useless bits of glass. Now you have a pinhole! No refraction necessary at all, but shrpness will deteriorate both by stopping down further (from diffraction) and by opening up (from the dreadful focusing capabilities of air).

    So to make a good pinhole, it should be small. The "circle of confusion" is directly proportional to the aperture of the pinhole, so smaller is better. But every hole has an edge, and edges give diffracton. So the closer together the edges are, the more diffraction will be visible - because the total amount of lihgt entering the hole gets smaller, and a larger proportion of it gets diffracted.

    There are lots of recommendations for the optimum size of a pinhole, unfortunately they have all fallen through a hole in my memory...
     
  16. Ed Sukach

    Ed Sukach Member

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    Not true. Consider the angles involved. If the light rays did not have to be "bent" at some point in the process, the only way an image could be formed would be if the lens - uh .. "pinhole" was at the same distance from the subject as it was from the "plane of ... uh... "film" - at a 1:1 ratio.. then all rays would be -- un-refracted is the best term.

    There are a lot of optical characteristics that are affected by a decreased aperture, depth of focus, being one - but not necessarily "resolution". Not all lens resolution is improved by reducing the aperture, however ... It all depends on the design. A *very* fast lens will provide *optimum* resoultion at apertures nearer the most wide open, as that is where performance is expected and where it was designed to be .... "general use" lenses, near the "middle" apertures and certain "process" and enlarging lenses - close to the smallest apertures.

    It is awfully difficult to describe all this in "three words or less". It is an interesting task to become familar with the "old days" of lens design - before computers, when lens manufactures had *crews* of people involved in "ray tracing" and calculating where the plane of focus would be - and curvature of focus - and so many other characteristics. Let me tell you about "Snell's law" and indices of refraction....
     
  17. Ole

    Ole Moderator Staff Member Moderator

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    Ed,
    a thorough understanding of refraction, birefringence, Snell's law, diffusion etc. were an important part of my education in Mineralogy.

    I think you're wrong :wink:
     
  18. Lemastre

    Lemastre Member

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    I fear my suggestion was not clearly stated. It was just a way to measure the diameter of the pinhole by blowing it up large enough to measure with a millimeter scale so that measurement could be compared with another item of known size blown up the same amount. Once you know the actual diameter of your pinhole, you can figure its f stop value. A pinhole's diffraction properties don't affect its ability to be projected.
     
  19. edbuffaloe

    edbuffaloe Member

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  20. rbiemer

    rbiemer Member

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    I've found that using very tiny drill bits--the numbered sizes(60 through80) is the best way to get a specific sized pinhole. I never could find a way to get reliable size info on sewing needles other than putting a micrometer on them. and I always ended up buying more than I really needed. I also reccomend Eric Renner's book very highly!
    Rob